Method and means for chemical analysis



Feb. 22, 1955 F. BLOCH ETA!- 3,

METHOD AND mus FOR CHEMICAL ,musxs BY NUCLEAR moucuons Original FiledDec. 2a, 1946 2 Sheets-Sheet 1 HUD/O HMPL lF/ER COIL 2 Dl-Q'PLACED R FMPLIFIE'R INVENTORS FELIX .9400

60 CYCLE SUPPLY N 05 M Y m w T 6 J L L 3 METHOD AND MEANS FOR CHEMICALAN ALY- SIS BY NUCLEAR INDUCTIONS Felix'Bloch, Palo Alto, Calif., andWilliam W. Hansen, deceased, late of Stanford University, Califi, byOlive D. Ross, executrix, Palo Alto, Calif., assignors of onehalf tosaid Felix Bloch and one-half to James L. Hansen, Palo Alto, Calif.

Original No. 2,561,489, dated July 24, 1951, Serial No. 718,092,December 23, 1946. Application for reissue July 22, 1952, Serial No.300,364

61 Claims- (Cl. 324-35) Matter enclosed in heavy brackets [II appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

This invention relates to the detection and identification of particularkinds of atoms, and more specifically to electrical methods foraccomplishing this result.

The objects of this invention are to provide method and means for rapidqualitative and quantitative chemical analysis. To provide method andmeans for analyzing for chemical elements without the destruction of thesample. To provide method and means for analyzing for a large number ofchemical elements in a single sample. To provide method and means fordifferentiating between the isotopes of a single element. To providemethod and means for measuring the precession frequency of the nuclei orportions of atoms in a prescribed magnetic field. To provide means forestablishing resonance between the precession frequency of the nuclei orportions of the atoms in a substance and the oscillations of a radiofrequency oscillator. To provide coupling by means of nuclear precessionbetween a coil producing an alternating magnetic field, and another coiloriented at right angles to said first coil. T provide coupling by meansof the precession of portions of atoms between translating meansproviding an alternating magnetic field and another translating meansoriented at right angles to said first translating means. To providemeans for accomplishing a cyclic variation of the precession frequencyof the nuclei relative to the oscillation frequency of the coil, andmeans for indicating the cyclic coincidence of the two frequencies. Toprovide means for modulating the frequency of the precession of thenuclei so that it will periodically coincide with the frequency in acoil, and to provide means for indicating said coincidence. To providemeans for modulating the frequency in a coil so as to periodicallycoincide with the frequency of precession of prescribed nuclei in aprescribed magnetic field, and means for indicating said coincidence.

To provide means for increasing the coupling between the precession ofthe nuclei and the thermal motion of the medium in which the nucleiexist. To provide means for reducing the coupling of two coilsapproximately at right angles to each other to a minimum. To providemeans for measuring the presence of paramagnetic molecules by theireffect on the damping of nuclear precession of another type of. atom. Toprovide means for tracing the course of certain reactions involving achange of valence, in which the element being traced is paramagnetic inone valence and diamagnetic in another. To provide means for theaccurate measurement of magnetic fields.

In order to explain this invention, it is first necessary to acquaintthe reader with a few of the known facts about the structure of theatom, and for this purpose we present the following rather briefstatement. More extended treatment can be found in any text on atomictheory: the one that follows merely states the facts without adducingproof and omits many important features not of interest here.

Any atom consists of a small heavy positively charged center, called thenucleus, surrounded by a relatively extensive ditfuse cloud ofelectrons. Normally, the total United States Patent fit ice

charge of the atom is zero so that the nucleus carries a positive chargeequal to the negative charge of the external electrons. The nucleus thencontains all of the positive charge and most of the mass associated withany atom. When one wishes to specify these quantities they are usuallymeasured in terms of the charge on the electron, e and the mass of aproton, or hydrogen nucleus. with atomic number 2'. and mass number Mhaving a positive charge Ze and a mass M times the 'mass of a proton.

Under all ordinary circumstances the atoms interactions with theexternal world occur by way of its ex ternal cloud of electrons and sothe arrangement of these electrons determines the gross, or chemicalproperties of the atom. Thus oxygen and nitrogen have different chemicalproperties because their external electrons are differently arranged.

This arrangement depends, naturally enough, on the number of electronsper atom. This number depends in turn on the nuclear charge Z so that ifwe know the nuclear charge and so the number of electrons we know thatmost of them have mass 16, and a few have masses l7 and 18. Such atoms,with the same number of elec-.

trons and so the same chemical properties, but different masses, arecalled isotopes.

Generally speaking, the mass numbers are roughly twice the correspondingatomic numbers, and the number of isotopes for a given Z increases as Zincreases.

That is, for the lighter chemical elements there are for I any given Zonly a few possible values of M while for the heavier chemical elementsa number of different masses are possible for a given Z.

Ordinarily, the various isotopes occur in nature in fixed proportions.For example, chlorine has two isotopes of masses 35 and 37 and thesealways occur in a mixture 812d; 5as5 to give a mean mass, or chemicalatomic weight, 0

In addition to charge and mass, discussed above, the nucleus may haveother properties. Two important ones that have been discovered are spinand magnetic moment.

It has been known for some time that the nuclei may have spin or angularmomentum i. e. may act like a small gyroscope. Part of this angularmomentum comes from the angular momentum, or spin, of the elementaryparticles which go to make up the nucleus, and part comes from therevolution of the component particles about the center of mass. Thisangular momentum is closely related to the spin of the electrons whichrotate about each nucleus and in addition spin upon their axes and actas small gyromagnetic resonators. For present purposes, we need not beconcerned with the rather imperfectly understood details of theintra-nuclear motions; all we need to know is that the nucleus as awhole has a total angular momentum. By well verified quantum laws, thisangular momentum must be a multiple of h/21r where h is Planck'sconstant. If we write the angular momentum as ill/21f then I may havethe values 0, 1/2, 1,

3/2 etc., but intermediate values never occur. It should be here pointedout that a particular species of nucleus wilth a particular value of Mand 2'. never changes its va ue.

This nuclear angular momentum or spin has been measured for a number ofnuclei and it is found that while I might, in principle, have anyintegral or half integral value, no value greater than 9/2 has even beenobserved. It follows that with only ten spin values and some hundreds ofnuclei, in general many different nuclei can be found having a givenspin value. No simple relation between Z and/or M and the spin has beenfound, except that nuclei with even M often have zero spin. Thus, onecannot predict from Z and M the spin value of an atom. Its value must bemeasured.

One might expect that a body, such as a nucleus, containing chargedparticles which are known to have a total angular momentum, might alsoexhibit magnetic prop- The usual symbols are Z and M, a nucleus If, for

i l l.

p. exhibited by the nucleus. nuclear magnetron eh/4w-Mc with e thecharge on the change is in the orientation.

It erties due to the motion of the charged particles. This expectationis verified and nuclei in general behave as if they containedcirculating currents. T he magnitude of such currents is best measuredby the magnetic moment A convenient unit is the electron, h Planck'sconstant, M the proton mass, and c the velocity of light.

A number of nuclear magnetic moments have been measured and it isobserved that: (a) if the spin is zero the magnetic moment is likewise(b) although only integral or half integral values of I are possible,any value of ,1. may occur and (e) no simple general relation between Z,M, I and pt has been found.

Thus any given nucleus has at least four properties; its charge, whichdetermines the chemical properties of the associated atom; its mass; itsspin or gyroscopic moment; and its magnetic moment.

Now a nucleus with given Z and M values will also I have definite valuesof I and ,u and these values of I and u, if they can be determined, willdetermine 2 and M values so that a method of measuring 1 and [.t is, ingeneral, a method of determining Z and so is a method of chemicalanalysis. An important exception occurs when I and p. are zero for thenthe atomic species is not determined. Practically this case does notoccur very frequently because most elements are mixtures of variousisotopes and usually at least one isotope will have a non zero spin andmoment.

The present invention is a method of measuring From them Z can be foundand so chemical analysis may be accomplished.

Before describing the present invention in detail, it will be useful toexplain the behavior of nuclei or other systems such as electrons withangular momentum under the influence of torques.

Consider first What happens when a nucleus is placed in a constantmagnetic field H which we will suppose to be in the vertical direction.Since the nucleus will generally have a magnetic moment, just as acompass needle has magnetic moment, one would at first expect theunclear moment to line up with the applied magnetic field, just as themoment of the compass needle lines up with the earths field. Actually,this is usually what happens eventually, but the process is more complexthan appears at first sight.

This complexity arises because the nuclear magnetic moment is associatedwith a mechanical angular momentum so that gyroscopic effects arise.

Since this angular momentum is a multiple of Plancks quantum constant,one might expect that a quantum mechanical treatment would be needed butit has been shown by Kramer's that in systems of the type hereconsidered which have only one spin and one momentum value, the quantummechanical and the classical treatments must always lead to the sameresult and so one may without prejudice, use either description. Wechoose here to use the language of classical mechanics since it is morefamiliar to the general reader.

Returning then to the case of a system with angular momentum acted on bya torque caused by the inter action between the nuclear magnetic momentand the external magnetic field, we observe that since the mag nitude ofthe angular momentum is fixed the only possible This orientation of theangular momentum vector changes steadily but always maintains a constantangle with the magnetic field so that the momentum vector moves on thesurface of a cone with axis parallel to the magnetic field. This motionis commonly called precession and the reasons for it and the equationsgoverning it may be found in any treatise on gyroscopes.

The problem is, in fact, exactly similar to that of a gyroscope acted onby gravity in which case it is well known that the gyroscope does notfall over but processes with the axis making a constant angle with thevertical.

In the absence of friction or other damping, this precession wouldcontinue indefinitely, and the gyroscope would never be oriented by thetorque due to gravity.

The angular rate at which this precession occurs depends on the torqueapplied and the angular momentum iii) all.

being larger for larger torques and for smaller angular momenta. Thus,in the nuclear case the rate of precession is proportional to ,uH/l itsactual value being (e/lZMcX li/l). lt will be observed that thisfrequency is independent of the angle which the nuclear moment makeswith the field M. This frequency of precession is often called theLarrnor frequency.

Thus we see that, in the absence of damping forces, nuclei when placedin a magnetic field would not line up therewith but would processcontinually about the axis established by the magnetic field. Actuallydamping forces do exist, as has been discovered by the presentinventors, and just as friction on a gyroscope eventually causes it toassume a position of lowest potential energy in the gravitational field,i. e. line up with the field, so these damping forces eventuallysuppress the nuclear precession and so allow the nuclear moments to lineup with the magnetic field.

A quantity of great importance, for present purposes, is the timerequired for these damping forces to act and this time we call therelaxation time. Experiment shows that this time may have valuesextending from 10* seconds or less to many minutes or more.

in addition to the gyroscopic and magnetic forces, and the dampingforces, as described above, there exists a third type of effect ofimportance; namely, that due to thermal motion. it is well known that asa result of such motions, the probability that a system in equilibriumwith its surroundings at absolute temperature T will have an energy E isproportional to U' where k is Boltzmans constant. As a result it is mostprobable that any given system will choose, for example, the lowest oftwo possible energies. The difierence in probabilities will be small itthe energy difference is small compared to id" and vice versa.

in the case of present interest the energy that the nucleus can gain byaligning itself with the magnetic field is of the order H and, onsubstituting numbers it will be found that this is about a million timesless than kT. As a result, although any nucleus is most stable whenlined up with the magnetic field, and although the damping forces permitit to so line up, the resulting stability is so small compared to theenergy ltT of thermal agitation that the orientation is determinedmostly by chance and only slightly by the magnetic torque between thenuclear moment and the magnetic field. Thus if we considered i,000,00lnuclei of spin 1:1/2 in a magnetic field of 1000 gauss or thereabouts wemight find 500,001 pointed with the field and 500,000 pointed against.The exact value of the difference depends, of course, on the moments,fields and temperatures involved, but the above numbers arerepresentative.

it might be thought that the description of the behavior of an ensemblecontaining 500,000 nuclei pointed one way and 500,001 pointed theopposite way would be quite complex. Actually it is surprisingly simple,for all the nuclei process at the same rate so that one can cancel the500,000 moments pointed against the field by 500,000 of the 500,001oppositely oriented but similarly processing nuclei. The end result isexactly the same as though there was a single nucleus aligned with thefield. the others simply cancelling oil.

To summarize: The combined effect of the nuclear angular momentum andthe torque exerted by a magnetic field and on the nuclear magneticmoment is to cause the nucleus to process at a frequency called theLarrnor frcqueney. This precession would continue indefinitely were itnot for damping forces which allow the nucleus to line up with thefield. The time required for this alignment is called the relaxationtime. Finally, because of large thermal forces which tend to give randomorientations to the nuclei, the total orientation achieved by thetorques acting on the nuclear moments is quite small.

We may also state that, with the exception of the effects of thermalforces, all the things we have described as happening to nuclei in amagnetic field in consequence of its angular momentum and the torque dueto its mag netic moment are also known to happen to an ordinarygyroscope when acted on by similar torques. In fact one might bycombining a gyroscope and a bar magnet maite a model which would exhibitall the phenomena of present interest.

Let us next consider a slightly more complex case in which, in additionto the constant magnetic field H in the vertical or Z direction, thereis a magnetic field Hi in) at right angles to Ho and with orientationwhich rotates about Ho with uniform angular velocity to. in general, wewill suppose that H1 is much less than H0.

Moreover, to simplify the treatment, we will assume that the nuclei havebeen in the field long enough so that any transient terms in the motionhave been damped out, i. e., they have settled down to their finalsteady motion.

This motion can be found by setting up suitable differential equationsand solving them, but it. can also be obtained in an instructivequalitative manner, as follows:

In the first place, since all transients have damped out, any cyclicmotion of the nuclei must occur with the angular frequency w of theimpressed rotating field H1. Thus if the nuclear angular momentum vectorrotates, it must do so at frequency w and must therefore rotate insynchronism with the field H1. Now, since We are searching for a stateof steady motion, no work can be done on the nucleus as otherwise itsenergy would vary.

This means that ,u., H1, and He must lie in the same plane since thenthe torque between and Hi will do no work. Thus we have a state in whichthe nuclear angular momentum vector processes about He, always lying inthe plane defined by He and the rotating Hi.

The nuclear angular momentum thus makes a constant angle, which we call6, with the .2". axis defined by Ho. To complete the description we mustsay what determines this angle.

It is apparent that two torques act on the nucleus, the one due to theinteraction between a and Ho, and the other due to ,u. and H1. Moreover,when 0 is small, the former is nearly zero while the latter is amaximum. 0n the other hand, when 0:90" and ,u. is lined up with E1, thetorque due to H0 is a maximum and that due to H1 is zero. Also, when 6varies, the rate of change of angular momentum, for a given rate ofprecession, varies.

What happens then, is this. The nucleus adopts an angle 6 such that thecombined torques due to Ho and Hi give a rate of precession just equalto w. in this way the nucleus is able to fulfill the assumption diat itwould process at a rate just equal to that of the field H1.

Or, to make a mechanical analogy which is more readily grasped, thecombined action of all the nuclei is equivalent to a single gyroscopewith a bar magnet for a shaft as shown in Fig. 1. The bar magnet issubjected to two torques acting in the plane defined by one of thetorques and the axis of the bar magnet. Since the bar magnet and bothtorques are in the same plane, the two torques must add to or subtractfrom each other, and since these two torques vary in relative magnitudedepending on the angle of the axis of the bar magnet, 21 very wide rangeof resultant torques are possible corresponding to a wide range ofprecession rates. Hence there is always an angle of inclination of thebar magnet which will give a precession rate in agreement with any rateof rotation of the magnetic field Hi.

This is one of two possible steady motions. As a little considerationwill show. the other may be obtained by simply reversing the orientationof the nucleus.

This is so because in making this change the directions of the torquesof both magnetic fields are reversed, but their magnitudes areunchanged. Hence their resultant is reversed in sign but unchanged inmagnitude, but since the gyroscope is turned over, this change of signdoes not change the direction of precession.

Up to this point it has been merely stated that the field H1 is arotating magnetic field. Such a rotating magnetic field could beestablished in the same way as a rotating magnetic field is establishedin a polyphase electric motor, but in practice it is usuallyinconvenient to do this. Actually a sinusoidally alternating magneticfield such as providcdby coils ll of Fig. l is all that is necessary,since such a field is equivalent to two magnetic fields of equalstrength rotating in opposite directions. The component rotating in thedirection of nuclear precession will produce the results describedabove, whereas the component going in the opposite direction willproduce no result since twice in every rotation the direction of theapplied torque reverses effecting complete cancellation.

In what follows we will, for simplicity, assume: (a) thatSlll'fiClGI'llL time has elapsed for the steady motion to beestablished, (b) that we consider only the excess nu clei oriented withthe field and (c) that any variations in w hereafter described takeplace in a time short compared to the relaxation time. Assimi ation {b}is always while all variations in w are made in a time short compared tothe relaxation time. These assumptions are by no means always true, butwhen they are not, theapparatus may still work, though in a slightlymore complex manner. Descriptions of the detailed behavior under suchcircumstances may be found in papers to be cited, but we can give hereSllfilCiEllt information for practical purposes without use of excessivemathematics. The two main efi'ects are: {a} the semi angle of theprecsssion cone 0 may not follow variations in m with perfect fidelityand (b) the nuclear moments will no longer lie in the rotating planedefined by Ho and H1. While taking these possibilitie into accountintroduces considerable mathematical complexity, the main practicalpoints are not altered, namely that the nuclei can be forced to processand that as willbe explained presently, this precession can be detectedby the voltage induced by the rotating magnetic moments.

It will be seen from the above that the relaxation time plays animportant role and it is therefore important to state that we havediscovered that this time may be varied over a wide range by introducingwhat we call a catalyst. Such catalysts greatly alter the rate at whichequilibrium is established, but do not participate in any other way. Anyatom, molecule, ion, etc. possessing a permanent magnetic moment in theelectron part of the atom, 1. a catalyst. For example, we have usedparamagnetic salts of iron and manganese and have also used dissolvedoxygen.

To return new to the behavior of processing nuclei,

we have seen that the angle 6 is established by a balance between theeifccts of Ho and Hi which gives a precession at angular rate (a. Theangle 9 may therefore be expected to depend on to, He, and H1. Supposefirst that w is very small so that the nuclei must process slowly. Thismeans a small torque which is achieved by a small value of 0 whichcorresponds to the magnetic moment t being almost lined up with thestrong field Hu. Now it we increase to, 0 will increase also until whenw=w0 we wish the moments to precess at the Larrnor frequency. But thisis exactly the frequency they would have under the intlucnceof Ho alone;we therefore find the nuclei lined up with H1 so that H1 produces notorque. Thus when ozwu the nuclei point at right angles to Hu. to givethe faster precession may be had by making 0 greater than 90 so thatboth Ho and H1 contribute torque. As 0 continues to increase, so does 0and finally, for values of w notably higher than wo the nuclei pointagainst Ho, i. e., they have been turned over." and at low values of w,the nuclei will again point with the field.

Likewise, it will be found that if we start with the nuclei pointed withH0, and a high value of w, that lowering through the Larmor value wewill turn the nuclei against HQ, the process being reversed as w isreturned to its original high value.

Entirely similar effects may be obtained by varying wt] instead of to,this being done by varying Ho.

We are now in a position to understand a detailed description of theapparatus of the present invention and the functioning thereof.

Referring to the drawings:

Fig. l is a schematic drawing illustrating the gyroscopic precession ofan atomic nucleus.

Fig. 2 is a vector diagram also explanatory of the precession of thenucleus.

Fig. 3 is a circuit diagram of the apparatus.

Fig. 4 is a cross section of the pickup head showing a part of themagnet poles and coils.

Fig. 5 is a schematic view to explain the action of the trimmer paddle.

Pig. 6 is a cross section of the electro-rnagnet with the pickup head inplace.

Referring now to Fig. 1, there are three sets of coils-- l, to, and 2respectively with mutually perpendicular axes.

Coil Z. is shown to one side of the drawing for convenience, but itsactual position is with axis perpendicular e., any paramagneticsubstance may act as it we increase in still further, more torque if nowan is decreased, the process will reverse r to the plane of the drawingand with the axis intersecting this plane in the region between coils tand 18.

Such axes are shown in Pig. 2 where the axes l and 18 are parallel tocoils it and 18. Coil 2 would then have its axis parallel to 19 which isat right angles to both 1' and 18'.

Also shown in Fig. 1 is a gyroscope l which represents the angularmomentum of a nucleus and, in dotted lines, a precession cone 21.

The coils 18 carry a steady or slowly varying current andproducc themagnetic field previously designated as 1-10. It is often convenient toprovide iron cores and return yoke for these coils. The coils 1 aresupplied with R. P. current of angular frequency to where by angularfrequency we mean 21r times the frequency in cycles per second. Thisalternating current produces an cillating magnetic field of magnitude2H1 cos art. This oscillating magnetic field may be considered as thesum of two rotating fields, just as may be done. for example,

in considering the field in a single phase induction motor.

As has been shown by Bloch and Siegert for another but similar problem.we need consider only the compo nent which rotates in the same directionas the Larmor precession, the other rotating componct produces rapidlyoscillating torques on the nuclei and these average to zero.

The atoms whose nuclei are to be studied are placed in the fields due tocoils lltl and l whereupon the rotating field produced by coils 1 causesthe nuclei to precess in the field i-lo due to coils 18. V

The processing nuclei, because of their magnetic moment, constitute arotating magnet and this rotating magnet induces voltages in coil 2whose axis is at right angles to coils l and 18. This voltage may beamplified by a suitable amount and measured, its magnitude constitutinga measure of the magnitude of the nuclear moments.

We may explain further by reference to Fig. 2 which shows mutuallyperpendicular axes in space 1', l8, and 19, 1' being parallel to theaxis of coils ll, 18' being parallel to the magnetic field produced bycoils 18 and 19 being parallel to the axis of pickup coil 2. The nucleithen precess about the field H0 along the axis 18 under the influence ofa rotating component of an oscillating field in the direction it. Theirprecession causes an oscillating field in the direction l9 and so induces a voltage in coils 2.

The magnitude of the oscillating field, and the voltage resultingtherefrom, depends on the angle between the processing nuclei and theaxis 18' and this in turn depends, as explained above, on the relationbetween the angular frequency or with which the driving field rotatesand the angular Larmor frequency wt). When the dr1ving field due tocoils 1 has frequency nearly equal to the Larmor frequency the nucleirevolve nearly in the I plane defined by l and '19 and the voltage incoil 2 is large, when the angular frequency w is far from can thevoltage in coil 2 is small. 7

Further practical details may be understood with the help of Figs. 3 and4- which relate to an apparatus used in tests of the detection methodhere described.

Fig. 3 is a block diagram and shows diagrammatically the poles of aniron core magnet producing field in the direction hitherto described as18'. This field is varied cyclically by a cycle current passing fromsource 8 through coils '7 and resistor 20. in this manner H and so we isvaried relative to the driving frequency w. Voltage across resistor 20is applied to the horizontal plates of cathode ray tube 9. Thehorizontal deflection of the cathode ray beam is thus proportional tothe deviation of H0 or we from the mean value determined by the steadyfield due to poles l0 and the magnet coils appertaining thereto. Radiofrequency power of angular frequency w is supplied by a transmitter 3 toan apparatus consisting of coils 7. and 1 and associated shields anddesignated as an R. F. head. Inside this R. F. head nuclei precess inthe manner described previously and induce a voltage which is led toamplifier 4 which increases its magnitude sufficiently to operatedetector 5. the output of which is proportional to the magnitude of theR. F. voltage supplied by 4. This voltage varies, rising to a maximumwhen we is equal to w and decreas' ing at other times. These variationsare amplified by audio amplifier 6 and finally govern the verticaldeflection of the spot of cathode ray tube 9.

ill]

Thus as the field HQ and so we is varied, the cathode ray spot moveshorizontally while, at the same time, the vertical deflection of thespot is a measure of the magni' tude of the voltage induced by theprecessing nuclei. Thus a curve of voltage induced as a function of weis traced out on the cathode ray screen. This process being repeatedcyclically 60 times a second; persistence of vision causes the curve toappear as fixed and stationary on the cathode ray screen.

In this'way the field giving maximum voltage, and so the field giving wow may easily be found.

Likewise by suitable apparatus not shown, but obvious to anyone skilledin the art, the magnitude of the voltage entering 4 may be measured andso the magnitude of the processing moment may be determined.

Referring now to Fig. 4 we may describe the contents of the R. F. head.The coil 1 is s own, it being Wound on the inside of a form 22. Thecurrent for this coil is carried by leads 23. inside this coil is glassbulb ill which contains the sample of material under study. The fieldproduced by the rotating held in sample ill induces volt ages in coil 2and this voltage is carried out for observa tion by means of a coaxialline with outer conductor 14 and inner conductor l3. The whole issurrounded by a shield 15, ill, which is split at suitable points toavoid 60 cycle eddy currents due to coils '7. so-cal'led paddle whichconsists of a more or less semicircular cap of copper rotatable by shaft17.

The function of paddle 12 may be-explained with the help of Fig. 5.

Although coils it and 2 are nominally at right angles it is inevitablethat, because of slight errors in construction, this perpendicularitywill not be perfect and as a result some flux from coil It may link coil2. Usually this leakage fiux will be many times the flux due to theprocessing nuclei and so the leakage flux may mask the effects due tothe nuclei.

This leakage illllt may be regulated by means of paddle 12. Being of agood conductor, such as copper, this paddle has induced in its eddycurrents which prevent the flux from penetrating the paddle. Thus, Fig.5A shows qualitatively the field due to coil 1 without the paddle whileFig. 5B shows the flux with the paddle in one position and Fig. 5C showsit in a diametrically opposite position. it is apparent that, by meansof the paddle, the flux lines inside ll may be made to slope either upor down. Thus the flux may be made to link coil 2 in either directionand it must therefore be possible by suitably orienting paddle T2 togreatly reduce the flux linkage coil 2.

it is found that the limit in their reduction is set by the finiteconductivity of the copper of which the paddle 15 made. As a result ofthis finite conductivity the currents induced in the paddle are notquite out of phase with the currents in coil ll. As a result whilefields of the same phase as the currents in coil i may be reduced tozero, a small field in time quadrature therewith will remam. Thusalthough the voltage induced in coil 2 can be reduced enormously it willnot in general be possible to make it zero by means of the apparatusshown.

This slight leakage from transmitter to receiver is not.

a handicap. in fact if it were not naturally left by the paddle onewould usually devise other means for introducing a small signal.

There are two reasons for this.

First, it is well known in the art that detectors, such as 5. are veryinefiicient and have a poor signal noise ratio when operated at a verylow level for they are then in the square law region. The voltagesinduced by the nuclei are small and would usuilly lead to operation inthe undesirable square law range. This is avoided by adding a constantleakage from the transmitter, the total voltage then being sufficient tooperate the detector in the linear range.

Second, the addition of a relatively large leakage to the small voltagedue to the nuclei has the result that the only variations in nuclearsignal that influence the detector are variations having a componentthat is in phase with the leakage signal, since variations in quadraturecompo nent only cause second order variations in the amplitude of signalfed to the detector. Thus by varying the phase of the leakage signal onecan study at will either the nuclear voltage that is in phase with thecurrent in coil 1 or that which is in quadrature therewith.

For most purposes, the residual leakage left by the sim- Also providedis a ple paddle arrangement above described is of the most interestingphase and has about the right magnitude.

When this is not so leakage of any desired magnitude and phase may beintroduced by connecting transmitter 3 and receiver 4 by means of aphase shifter and variable attenuator.

The earliest results obtained using the apparatus of Figs. 3 and 4 wasconcerned with the nuclear induction effect for hydrogen. Hydrogen is ahighly satisfactory substance to use for initial investigations becauseit has the highest gyromagnetic ratio of any atom and a highconcentration of hydrogen is present in water and many organicSubstances. These two facts insure a strong signal with relatively weakmagnetic polarizing fields. The first signals were obtained from asample containing only c. c. of water. Samples were later increased to 1c. c.

One striking demonstration of clear resonance is obtained by raising thesteady field slightly above the resonance point and then turning it off.The field in a large magnet dies somewhat gradually when the current isturned off, and the signal may be seen to suddenly appear as themagnetic field passes through the resonant value, or the value for whichthe axes of the nuclei are precessing at 90 to the magnetic field.Numerous other variations of the method of operation all gave resultsconsistent with the theory. Work also has been done on the heavyhydrogen, fluorine, etc., with equally satisfactory results.

Finally, in Fig. 6 we show the apparatus as it appears set up in thelaboratory. Here 24 is a large electromagnet which replaces thediagrammatic coils 18 and the R. F. head is shown between its poles 10.It is clear after a moments thought that if magnet 24 has a field ofunknown strength, that field can be measured by use of a samplecontaining known atoms, whereas if the atoms are known, they can beidentified if the field strength is known.

To sum up the operation of the present invention as a means of chemicalanalysis: The substance under investigation is placed in an apparatus asdescribed and the nuclei caused to precess under the combined action ofa steady field H and an R. F. field of angular frequency w. The voltageinduced by this precession reaches a maximum when w and the Larmorfrequency can approach coincidence. By finding this maximum the Larmorfrequency is determined. This Larmor frequency is proportional to uHo/iand, knowing Ho, a/l can be found. Since different nuclei have differentvalues of /I a determination of this quantity identifies the nuclearspecies present. Measurements of intensity may also be made to determinethe number of nuclei present.

It will be apparent from the many statements made above about nuclearspins and moments that these quantities have been measured by othersbefore the present inventors introduced the device here described. Themethods used have been quite varied, but the only one, to our knowledge,which has any similarity with the present method and which was known tothe art at the time the present inventors had completed their work isthat exploited by Rabi and his co-workers. Rabis methods may be founddescribed in various articles, of which we may mention one in TheReviews of Modern Physics, volume 18, page 323, of July 1946. Inconjunction with this article one may also read two articles by thepresent inventors which describe some of the theory in greatermathematical detail and also give details of experiments performed withthe apparatus of the present invention. The articles are to be found inthe Physical Review, volume 70, pages 460 through 484, of October 1946.

It is believed that a study of. the article by Rabi in conjunction withthe present description will reveal that there are very broaddifferences between the methods. We may point out at least four such.

First, the apparatus used in the two experiments is entirely different.

Second, in Rabis method the material is examined at very low density.Specifically, the material is examined in the gas or vapor phase and thepressure is of order of a hundred millionth of an atmosphere or less.

Third, our apparatus detects the nuclear moment directly by the voltageit induces, Rabis method records the effect of the nuclear moments indeviating a beam of atoms.

Fourth, we require an average polarization of our sample; such averagepolarization of the mass of material is not sought nor obtained by Rabi.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Atom identifying apparatus for operating on a sample of matter havinga predominant orientation of the magnetic poles of the nuclei thereof,comprising means applying a moving magnetic field for causing saidpredominant orientation to precess, and means for detecting saidprecession.

2. in nuclear identifying apparatus, means causing precession of apredominant orientation of the magnetic poles of nuclei, and means fordetecting the precession frequency of said magnetic poles of saidnuclei.

3. Apparatus for identifying the atomic constituents of a sample ofmatter comprising magnetic field establishing means causing the nucleiof the atoms to orient themselves in a predominant direction, magneticmeans for changing the angle of said predominant direction, and meansfor detecting said change.

4. In apparatus for analyzing a sample of matter, means for causing thenuclei of atoms to orient themselves in a predominant direction, meansfor causing said predominant direction of orientation to precess, andmeans for detecting the precession frequency of said nuclei.

5. In apparatus for identifying atoms, magnetic means for causing thenuclei of atoms to orient themselves with their magnetic poles in afirst direction, magnetic means for precessing said nuclei about saidfirst direction, and radio frequency means for detecting saidprecessing.

6. Means for identifying particular varieties of atoms in samples ofmatter having densities greater than one millionth of a gram per litercomprising magnetic means for causing precession of the gyroscopic axesof the nuclei of said atoms and means for detecting the precessionfrequency of said nuclei.

7. In apparatus for identifying particular kinds of atoms, means forcausing a preponderant orientation of the magnetic poles of nucleicomprising means generating a magnetic field, means for increasing thedamping of the precession of said nuclei, means including a source of amagnetic field for effecting a forced precession of said preponderantorientation, and induction means responsive to the magnetic moments ofsaid nuclei for indicating the magnitude of the voltage resulting fromthe precession of said magnetic poles.

8. Apparatus for identifying particular types of atoms comprising asource of a strong magnetic field uniform over the volume occupied bysaid atoms for orienting the atoms in a predominant direction, a sourceof an alternating magnetic field acting at right angles to said fieldfor causing precession of the atoms, and high frequency detecting meansresponsive to the magnetic moment of said atoms while under theinfluence of said alternating magnetic field.

9. Apparatus for identifying particular types of atoms comprising astrong magnetic field means uniform over the volume occupied by saidatoms, a rotating magnetic field. means with the axis thereof at rightangles to said strong magnetic field means, and high frequency detectingmeans responsive to magnetic fields from said atoms while under theinfluence of said rotating field.

10. in apparatus of the character described, electromagnetic means forcausing a predominant orientation of nuclei, electromagnetic means forcyclically reversing said predominant orientation, and radio frequencymeans responsive to magnetic fields set up by said nuclei for detectingthe occurrence of said reversal.

ll. In apparatus for determining species of atoms, magnetic means forcausing precession of the atoms about the direction of a magnetic fieldof a preponderant orientation of polarized nuclear magnetic moment at apredetermined precession rate, radio frequency detecting meansresponsive to said nuclear magnetic moment, cyclically varying means forchanging said precession rate, and regulating means for controlling thedetecting means in synchronism with said cyclic variation.

' 12. Apparatus for determining the ratio of the magnetic moment toangular momentum of the nuclei of atoms of a sample of matter,comprising means including a source of a cyclically varying magneticfield for polarizing the nuclear magnetic moment of a sample of matter,electromagnetic means for creating a magnetic neld at right angles tosaid polarization, electromagnetic detecting means responsive to theprecession of the magnetic moment of said nuclei, and regulating meansfor controlling the detecting means in synchronism with said firstcyclically varying magnetic field.

13. Apparatus for identifying atoms comprising means for varyingprecession angle of polarized-magneticmoment nuclei, and means forindicating said precession angle, said means for varying said precessionin cluding a coil for generating an alternating magnetic field, and saidindicating means including a coil substantially at right angles thereto.

14. Apparatus as in claim 13 including an adjustable metal paddle forslightly deflecting the field associated with one of said coils.

15. A method for identifying certain kinds of atoms which comprises thesteps of partially polarizing the nuclei of atoms by applying a magneticfield thereto, precessing the axis of said polarization by applicationof a second magnetic field at right angles to the first, and detectingthe presence of said precession.

16. A method for identifying atoms comprising the steps of partiallypolarizing the nuclei of atoms by application of a steady magnetic fieldhaving a predetermined direction, causing the axis of said polarizationto process by application of a rotating component of magnetic field atright angles to said direction, and detecting the presence of saidprecession by the effect of the magnetic moments of said atoms.

17. Apparatus for measuring the strength of a magnetic field comprisinga sample of matter possessing nuclei having a prescribed magneticmoment, electromagnetic means for subjecting said sample to a rotatingcomponent of magnetic field of controllable frequency, means forinserting said sample and said means into a magnetic field to bemeasured, and means for measuring the combined magnetic moment of saidprecessing nuclei as a function of said frequency.

18. Apparatus for measuring the strength of a magnetic field comprisinga sample of matter possessing nuclei having a prescribed magneticmoment, means including a source of a magnetic field for subjecting saidsample to a variable frequency rotating component of said field, and

means for detecting a 90 precession angle of a preponderant nuclearpolarization caused by said rotating component, and indicating meansresponsive to the frequency of said rotating component.

19. The method of measuring a magnetic field com prising the steps ofinsertion of a substance into said field. said substance having a knowngyromagnetic ratio, subjecting said substance to a rotating component ofmagnetic field of controllable frequency, measuring as a function ofsaid controllable frequency the precession of the polarized nuclearmoment of said substance. thereby determining the strength of themagnetic field.

20. The method of measuring the relationship of precession frequency ofatomic nuclei to the magnetic field in which precession is taking placewhich consists of the steps of precessing a polarization of nuclearmagnetic moment in a magnetic field and detecting said precessionfrequency.

21. A method as in claim 20 including the step of measuring theamplitude of the precession signal.

22. The method of identifying particular kinds of atoms which consistsin precessing the nuclei of a nuclearly polarized sample of matter in amagnetic field of known strength. and measuring the frequency of saidprecession.

23. A method as in claim 22 in which the amplitude of the signalproduced by the precessing nuclei is also measured to determine thequantity of a particular kind of atom present.

24. The method of measuring magnetic fields which consists of precessinga nuclearly polarized sample of known atoms in a magnetic field ofunknown strength. and measuring the frequency of nuclear precession insaid sample.

25. The method of measuring the relationship of the precession frequencyof atomic nuclei to the magnetic ill till

said reversal,

field in which precession is taking place which consists of the steps ofpolarizing the nuclear magnetic moment of a sample of matter, precessingsaid polarized moment, and detecting said precession frequency.

26. The method as described in claim 25 including the step of measuringthe amplitude of the signal produced by precession of thenuclei.

27. The method of identifying particular ltinds of atoms in a samplewhich comprises subjecting a sample of matter to a first magnetic fieldto polarize the magnetic moments of the nuclei thereof, in additionsubjecting said sample to a rotating component of magnetic field havinga component of rotation about the axis of said constant magnetic fieldto precess said polarized moments, and inductively measuring saidpreccssed polarized moments.

28. The method according to claim 27 with the added step of sinusoidallyvarying the first magnetic field and synchronously controlling thedetecting means so as to indicate the precise time in the cycle when thefrequency of precession of the nuclei in the first field is equal to therotation frequency of the rotating field.

29. The method of measuring the relationship of the precession frequencyof atomic nuclei in a sample of matter to the magnetic field in whichprecession is taking place which comprises the steps of polarizing thecom bined magnetic moment of said nuclei. reversing said polarization bymeans of a rotating component of magnetic field having a rotationfrequency, detecting the reversing process,- thereby determining therotation frequency of said rotating component for which said reversaltakes place.

30. The method as described in claim 29 with the additional step ofadding a paramagnetic substance to the sample to decrease the timerequired to produce nuclear polarization.

31. The method according to claim 29 with the added step of modulatingthe frequency of the rotating component of magnetic field, andsynchronously controlling the detecting to indicate the precise time inthe modulating cycle when the normal precession frequency in the steadymagnetic field coincides with the frequency of rotation of the rotatingfield component.

32. In apparatus for determining the ratio of the magnetic moment toangular momentum of a sample of matter. a source of a first magneticfield, a source of a second magnetic field having a component at rightangles to said first magnetic field, said sample of matter beingsubjected to said fields to produce torques thereon, modulating meansfor varying the magnitudes of said torques acting on the magneticmoments of the nuclei of said matter. detecting means responsive to themagnetic moment of said nuclei when the torque due to one of said fieldsis zero, whereby said ratio is determined.

33. In apparatus for identifying particular types of atoms, meansincluding a source of a magnetic field for orienting the nuclei of saidatoms in a predominant direction with respect to said first field, meansincluding a second source of an alternating magnetic field having anadjustable frequency for causing a cyclical variation in the orientationof said nuclei relative to said first field involving a reversal of saidorientation of said nuclei, and detecting means for indicating themagnitude of the combined magnetic moments of said nuclei at the instantof whereby the Larmor frequency is determined.

34. Apparatus for identifying atoms comprising means for subjecting saidatoms to a first magnetic field to effect the polarization of themagnetic moments of said atoms, further means for subjecting said atomsto an alternating magnetic field having a component at right angles tosaid first magnetic field to effect a variation in the precession an leof the magnetic moments of said atoms. and indicating means responsiveto said magnetic moments.

35. In atomic identifying apparatus for determining the species ofunknown atoms comprising means including a source of a first magneticfield. means including a source of a second magnetic field, one of saidmag 'netic fields having a rotating component normal to the magneticmoments of said nuclei for determining said reversal.

36. In apparatus for identifying an atomic sample by a determination ofthe ratio of the magnetic moment to the angular momentum of said samplecomprising a source of a first cyclically varying magnetic fieldincluding an energized magnet and a set of coils all beingconcentrically positioned about a first axis, and a substantially lowfrequency electromagnetic energy source connected to said set of coils;a source of a second cyclically varying magnetic field including a coilhaving a second axis positioned substantially normal to said first axis,and a transmitter having an adjustable frequency output; positioningmeans for locating said sample substantially at the intersection of saidaxes, said magnetic fields for polarizing the magnetic moment of saidsample in a predominant direction and precessing the sample about saidpredominant direction with a precession angle depending on the frequencyand magnitude of said fields; a pick-up coil having an axissubstantially at right angles to the aforementioned axes and furtherhaving a voltage induced therein as a function of the precessingmagnetic moment of said sample; indicating means including an amplifierand oscilloscope connected to said pick-up coil for providing a visualindication of the magnitude of said magnetic moment, the horizontalsweep of said oscilloscope being connected to and synchronized with saidlow frequency energy source connected to said set of coils, whereby saidratio is given in terms of said frequency and the phase of theoscilloscope indication relative to said low frequency said lowfrequency at the instant said magnetic moment is normal to saidpredominant direction.

37. In apparatus for identifying a sample of matter, means for orientingportions of the atoms of the sample of matter in a predominantdirection, means for causing said predominant direction of orientationof such portions to precess, and a detector for indicating the angularrate of precession.

38. In apparatus for analyzing a sample of matter, means for orientingportions of the atoms of the sample of matter in a predominantdirection, means for causing said predominant direction of orientationof the portions to precess at predetermined frequencies about apredetermined axis, and a detector for indicating the angular rate ofprecession about said axis.

39. Apparatus for analyzing a sample of matter having its constituentelements oriented in a predominant direction comprising means forcausing said predominant direction of orientation of the elements toprecess about a predetermined axis, and means for detecting the angularrate of precession with respect to said axis.

40. In combination, means for producing a unidirectional field, meansfor producing an alternating field substantially at right anglesthereto, and detecting means located adjacent to the intersection ofsaid unidirectional and alternating fields for indicating the fieldstrength at: right angles to said unidirectional and alternating fields.

41. The combination of claim 40, wherein said alternating fieldalternates at adjustable radio frequencies.

42. In combination, matter comprising atoms having portions possessingthe properties of magnetic moment and gyroscopic moment, means forproducing a unidirectional magnetic field enveloping said matter, energytranslating means for supplying an alternating magnetic field of radiofrequency for passing through said matter substantially at right anglesto said unidirectional field, said atom portions precessing in saidunidirectional field due to said radio frequency field and producing aradio frequency magnetic field at an angle to both said unidirectionalfield and said supplied alternating field, and additional energytranslating means located adjacent to the intersection of saidunidirectional field and said alternating magnetic fields and orientedat an angle to said first energy translating means.

43. In combination, matter comprising atoms having portions possessingthe properties of magnetic moment and gyroscopic moment, means forsupplying a unidirectional magnetic field enveloping the matter, radiofrequency energy translating means coupled to said matter for supplyingan alternating magnetic field of radio frequency passing into saidmatter substantially at right angles to said unidirectional field, saidatom portions precessing in said unidirectional field due to said radiofrequency field and producing a resultant radio frequency magnetic fieldextending at an angle to both said unidirectional field and saidsupplied alternoting field, and additional radio frequency energytranslating means coupled to the matter and oriented at an angle withrespect to the first energy translating means for carrying radiofrequency energy passing out from the matter after the energy passingthrough the matter has reacted with the processing atom portions in thematter. 7

44. In combination, matter comprising atoms having portions possessingthe properties of magnetic moment and gyroscopic moment, means forproducing a .uni-

directional magnetic field enveloping the matter, energy translatingmeans having a part thereof for conveying an alternating magnetic fieldof radio frequency into the matter, said alternating field extendingsubstantially at right angles to the unidirectional field for causingthe atom portions of the matter to precess in the unidirectional fielddue to the radio frequency field and thereby produce a radio frequencymagnetic field at an angle with respect to both the unidirectional fieldand the first alternating field, said energy translating means havinganother portion thereof for conveying radio frequency magnetic fieldenergy out from the matter.

45. Apparatus for measuring the strength of an unknown unidircctionalmagnetic field including matter comprising portions of atoms havingknown gyromagnetic properties, means for positioning the matter in theunknown field to thereby orient a preponderance of the atom portions inthe direction of the field, means for supplying a controllable radiofrequency magnetic field to the matter substantially at right angles tothe urn known field to cause the oriented atom portions to precessin theunknown field at the frequency of the applied radio frequency field, andelectromagnetic means for detecting the magnetic field produced by theprecessmg atom portions.

46. The method for measuring the strength of an unknown unidirectionalmagnetic field which comprises positioning a sample of matter comprisingportions of atoms having known gyromagnetic properties within theunknown field, supplying a controllable radio frequency alternatingfield to the matter substantially at right angles to the unknown field,detecting the preces sion of the portions of the atoms in the unknownfield due to the applied radio frequency field by picking up the radiofrequency magnetic field produced by the precessions, and determiningthe strength of the unknown field from the values of the frequency ofthe detected radio frequency field and the known gyromagnetic propertiesof the atom portions.

47. The method for identifying certain kinds of atoms which comprisesthe steps of polarizing atom portions possessuzg gyromagnetic propertiesby applying a unidirectional magnetic field thereto, applying a radiofrequency magnetic field thereto substantially at right angles to theunidirectional magnetic field to thereby precess the axis of saidpolarization, and detecting the radio frequency magnetic field set up bythe gyromognelic bodies precessing in the unidirectional magnetic field,the frequency of said detected precession serving to determine the kindof atom.

48. In combination, limiter comprising atoms" having portions possessingthe properties of magnetic moment and gyroscopic moment polarized in afield in which the portions may precess, energy translating means forsup plying an alternating magnetic field of radio frequency to saidmatter substantially at right angles to said polarizing field, said atomportions processing in said polarizing field dueto said radio frequencyfield and producing a radio frequency magnetic field at an angle to bothsaid polarizing field and said supplied alternating field, andadditional energy translating means located adjacent to the intersectionof said polarizing field and said alternating magnetic fields andoriented at an angle to said first energy translating means.

49. The method of obtaining an energy output from an ensemble ofportions of atoms possessing the properties of magnetic moment andgyroscopic moment polarized in a field in which the portions may precesscomprising the steps of applying an alternating magnetic field of radiofrequency energy to the matter having a field component at right anglesto said polarizing field to produce a radio frequency magnetic field atan angle to both said polarizing field and said applied alternatingfield, and detecting the magnetic field produced by the precessing atomportions.

50. Apparatus for producing periodic resonance between the frequency ofprecessions of portions of atoms possessing the properties of magneticmoment and gyroscopic moment and a radio frequency field applied to suchportions comprising, means for producing a unidirectional magnetic fieldfor enveloping a volume of matter containing the portions of atoms,means for producing a radio frequency magnetic field substantiallynormal to said unidirectional magnetic field to thereby cause saidportions to precess in the unidirectional field and means for providingan audio frequency field for modulating the unidirectional magneticfield.

51. The method of producing periodic resonance between the frequency ofprecession of portions of atoms having the properties of gyroscopicmoment and magnetic moment and a radio frequency field applied to theportions which comprises, the steps of positioning a volume of mattercontaining the portions of atoms within a unidirectional magnetic field,applying the radio frequency field to said matter substantially at rightangles to said unidirectional magnetic field to thereby cause saidportions to precess in said unidirectional magnetic field, andmodulating the unidirectional magnetic field with an audio frequencymagnetic field to thereby periodically sweep through the region ofresonance.

52. Apparatus for measuring the strength of a unidirectional magneticfield comprising a volume of matter containing portions of atomspossessing the properties of magnetic moment and gyroscopic momentadapted to be positioned in said unidirectional magnetic field, meansforproviding a variably controlled radio frequency magnetic field to thematter substantially normal to the unidirectional magnetic field tothereby cause said portions to precess in said unidirectional magneticfield at a frequency in resonance with the applied radio frequency fieldand means for modulating said unidirectional magnetic field with anaudio frequency magnetic field to thereby periodically sweep through theregion of resonance.

53. In combination with apparatus as claimed in claim 52, means forindicating resonance between the portions of atoms precessing in themodulated unidirectional magnetic field and the applied radio frequencyfield.

54. In combination, a volume of matter containing portions of atomshaving the properties of magnetic moment and gyroscopic moment, meansincluding a source of a first magnetic field enveloping the matter,means including a source of a second magnetic field enveloping thematter, one of said magnetic fields having a rotating component normalto the other field, at least one of said means polarizing the magneticmoments of the portions of atoms in a predominant direction, at leastone of said means being modulated for varying the precession angle ofthe portions including a reversal in said predominant direction, andmeans responsive to the magnetic moments of said portions fordetermining said variation.

55. In combination, means for positioning a volume of matter containingportions of atoms having the properties of magnetic moment andgyroscopic moment within a unidirectional magnetic field to therebypolarize the portions, means for providing a radio frequency magneticfield to the matter substantially at right angles to the unidirectionalfield to cause the portions to precess in the unidirectional field atthe frequency of the applied radio frequency field, and means formodulating at least one of the fields with an audio frequency.

56. Apparatus for producing resonance between the frequency ofprecession of portions of atoms having the properties of gyroscopicmoment and magnetic moment and a radio frequency field applied to theportions comprising means containing a volume of matter having saidportions of atoms adapted to be located in a unidirectional magneticfield. and means including a coil of wire for applying a radio frequencymagnetic field to said matter at an angle to said unidirectional fieldto thereby cause said portions to precess in said unidirectionalmagnetic field at a frequency in resonance with the applied radiofrequency field and for detecting the resonance,

5 7. A passive network comprising a polarized medium, input means tosupply an alternating voltage V1 to said medium to produce analternating current flow 11 in said input means, output means coupled tosaid medium to obtain an alternating voltage Vz producing a current flowI: in said, output means, said medium possessing gyromagneticcharacteristics and including material having an anisotropicpolarization property wherein:

where S is a constant.

58. A passive four-terminal network comprising a polarizable mediumpossessing gyromagnetic properties, input terminals to apply an inputvoltage and an input current to said network, output terminals to takean output voltage and an output current from said network, means topolarize said medium in a first direction, means connected to said inputterminals to produce a primary field in said medium in a seconddirection different from said first direction, said primary fieldproducing in said polarized medium a secondary field having a thirddirection different from said first and second directions, and

means coupled to said secondary field to produce said output voltage andoutput current at said output terminals. I

59. A circuit arrangement having a passive four-terminal networkcomprising a polarizable medium possessing gyromagnetic properties,input terminals to apply an input voltage and an input current to saidnetwork, output terminals to take an output voltage and an outputcurrent from said network, means to polarize said medium in a firstdirection, means connected to said input terminals to produce a primaryfield in said medium in a second direction different from said firstdirection, said primary field producing in said polarized medium asecondary field having a third direction difierent from said first andsecond directions, and means coupled to said secondary field to producesaid output voltage and output current at said output terminals, therelation between said input voltage V1, said input current I1, saidoutput voltage V2 and said output current I2 being wherein Z1: is theinput impedance of the network, Z2: is the output impedance of thenetwork and Z21 and Z1; are transfer impedances having unequalmagnitude.

60. A passive four-terminal network comprising a polarized mediumpossessing gyromagnetic properties, said medium being polarized in afirst direction, input terminals to apply an input voltage and an inputcurrent to said network, output terminals to take an output voltage andan output current from said network, means connected to said inputterminals to produce a primary field in said medium in a seconddirection difierent from said first direction, said primary fieldproducing in said p0- larized medium a secondary field having a thirddirection different from said first and second directions, and meanscoupled to said secondary field to produce said output voltage andoutput current at said output terminals.

61. In combination, a volume of matter containing portions of atomshaving the properties of magnetic moment and gy roscopic moment, meansproducing a polarizing magnetic field thereby determining a resonantfrequency of said atom portions, means for producing electric energy ofa prescribed frequency, means for coupling said energy to said atomportions, and means for periodically bringing the frequency of saidenergy producing means and the resonant frequency of said atom portionsinto coincidence.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS

